Collimated sputter deposition monitor using sheet resistance

ABSTRACT

A method and apparatus are disclosed for electrically monitoring processing variations of a material deposited using a collimated process. In one embodiment, the method and apparatus are directed to monitoring variations in step coverage of a conductive material deposited using a collimated sputtering process. A substrate having a plurality of trenches is used to mimic features desired to be monitored, such as contact holes. The resistance of metal deposited into the trenches is monitored to determine the effectiveness of the collimated sputtering process.

This application is a Continuation of application Ser. No. 08/788,980,filed Jan. 27, 1997 allowed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the field of integrated circuitmanufacturing and, more particularly, to a method for monitoringprocessing variations in sputtered metallic films.

2. Description of the Related Art

For many years, metal sputtering has been used to form films onsemiconductor wafers. Although sputtering is generally regarded as acost effective way to deposit metal on a semiconductor wafer, sputteringtechniques have suffered from technical drawbacks in theUltra-Large-Scale-Integration (ULSI) era. Most significantly, it isbecoming increasingly difficult to sputter metals into contact holes.The aspect ratio of a contact hole is determined by dividing the contacthole's height by its diameter. In the ULSI era, aspect ratios of contactholes are growing larger as the diameters shrink. As a result, it hasbecome increasingly more difficult to sputter metals, such as titanium,into the bottom of the contact holes. Inadequate metal formation at thebottom of a contact hole produces a contact with an undesirably highresistance.

A collimated sputtering process has been used to achieve betteruniformity at the bottom of contact holes because such a process directsthe deposited metal in a substantially perpendicular manner onto thesurface of the wafer. In a collimated sputtering process, a collimatoris placed between a metal target and a wafer. The collimator containsholes through which liberated atoms from the metal target must pass toreach the wafer. Atoms having trajectories substantially perpendicularto the wafer pass through the collimator, while atoms having undesiredtrajectories accumulate on the walls of the collimator.

While collimation provides for contacts with better coverage at thebottom of the contact holes, the technique does suffer from the drawbackthat the inside walls of the collimator become coated with the metalatoms. Over time, the build-up on the walls changes the effectivetrajectory of the atoms that are able to proceed to the wafer surfaceand affects the profile of the metal film so deposited. Once thisbuild-up becomes severe, the collimator is cleaned or changed. However,there is no method or apparatus to monitor the status of the collimatorto determine when to clean or change it.

The present invention is directed to overcoming, or at least reducingthe affect of, one or more of the problems set forth above.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention there is provideda method for fabricating a monitor for monitoring processing variationsof a conductive material. The method includes the steps of: (a) forminga plurality of trenches on a substrate; (b) disposing a conductivematerial into the plurality of trenches; (c) placing two terminals incontact with the conducting material; and (d) coupling the two terminalsto a resistance measuring device.

In accordance with another aspect of the present invention, there isprovided a method for monitoring step coverage of a conductive material.A substrate having a plurality of trenches is placed in a depositionchamber. A conductive material is disposed into the plurality oftrenches. Two terminals are placed in contact with the conductivematerial. A resistance is measured between the two terminals.

In accordance with a further aspect of the present invention, there isprovided a method for monitoring step coverage of a conductive materialto be deposited onto a production wafer containing contact holes havinga given aspect ratio. The production wafer is placed into a depositionchamber. A test wafer having a plurality of trenches is placed into thedeposition chamber. The trenches have an aspect ratio similar to theaspect ratio of the contact holes. Conductive material is disposed intothe trenches. Two terminals are coupled to the conductive material, anda resistance is measured between the two terminals.

In accordance with yet another aspect of the present invention, there isprovided an apparatus for monitoring processing variations of aconductive material relative to a given feature. The apparatus includesa substrate having a plurality of parallel trenches. The trenches havean aspect ratio correlative to a given aspect ratio of the givenfeature.

In accordance with still another aspect of the present invention, thereis provided an apparatus for monitoring processing variations of aconductive material relative to a given feature. The apparatus includesa substrate having a plurality of parallel trenches. The trenches havean aspect ratio correlative to a given aspect ratio of the givenfeature. The trenches have a conductive material disposed therein. Twoterminals are coupled to the conductive material.

In accordance with a still further aspect of the present invention,there is provided an apparatus for monitoring processing variations of aconductive material relative to a given feature. The apparatus includesa substrate having a plurality of parallel trenches. The trenches havean aspect ratio correlative to a given aspect ratio of the givenfeature. A conductive material is disposed in the trenches. Twoterminals are coupled to the conductive material, and a resistancemeasuring device is coupled to the two terminals.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages of the invention will become apparentupon reading the following detailed description and upon reference tothe drawings in which:

FIG. 1 is a cross section of a contact hole;

FIG. 2 is a cross section of a contact hole which is filled with metaland which has relatively poor metal coverage at the bottom of thecontact hole;

FIG. 3 is a schematic representation of a metal deposition machineshowing the location of a collimator;

FIG. 4 is a cross section of the target, collimator, and wafer;

FIG. 5 is a cross section of a contact hole which is filled with metalsputtered through a collimator and which exhibits better metal coverageat the bottom of the contact hole than FIG. 2;

FIG. 6 is a plan view of a wafer having a plurality of trenchedstructures;

FIG. 7 is a three-dimensional perspective view of the trenched structureused in the disclosed process;

FIG. 8 is a cross section of the trenched structure showing the locationof the test terminals which measure the resistance of the metalsputtered over the trenched structure;

FIG. 9 is a three-dimensional perspective view of an alternativeembodiment which allows for the monitoring of metal deposition where themetal's resistance is to be measured parallel to the trenches in thetest structure; and

FIG. 10 is a cross section of the structure in FIG. 8 after metaldeposition and removal of the metal from the top surfaces of thestructure by the use of a Chemical-Mechanical process (CMP).

While the invention is susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and will be described in detail herein. However,it should be understood that the invention is not intended to be limitedto the particular forms disclosed. Rather, the invention is to cover allmodifications, equivalents and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENTS

Referring to FIG. 1., a cross sectional view of a contact hole 10 isshown. The contact hole 10 has been etched, for example, into an oxidefilm 12 which has been deposited on a substrate, such as a semiconductorwafer 14. As mentioned previously, inadequate metal formation at thebottom of a contact hole produces a contact with an undesirably highresistance. An example of a contact 16 that exhibits poor metal coverageat the bottom of the contact hole 10 is shown in FIG. 2.

A collimated sputtering process may be used to achieve better uniformityof metal at the bottom of contact holes 10 because such a processdirects the deposited metal in a substantially perpendicular manner ontothe surface of the wafer 14. This process can best be understood withreference to FIG. 3, which shows the basic internal components of ametal sputtering chamber 20. Reactive gas is fed into the chamber 20. Aplasma is produced in the gas between a metal target 22 and the wafer 14by forming a sufficiently high electrical potential difference betweenthe metal target 22 and the wafer 14. This causes the ions produced inthe plasma to bombard into the metal target 22, thus liberating metalatoms from the metal target 22. The liberated metal atoms then fall ontothe surface of wafer 14.

A collimator 26 is placed between the metal target 22 and the wafer 24.The collimator 26 typically has a hexagonal, or “honey comb,” holepattern 28 built into it. The liberated atoms move through thecollimator 26 to reach the wafer 14. To understand how the collimatedprocess achieves a more uniform contact fill, one should first note thatthe various metallic atoms that are liberated from the metal target 22typically have different trajectories as a result of the relativelychaotic liberation process. The collimator 26 ensures that onlyliberated atoms that have trajectories substantially perpendicular towafer 14 will be deposited onto the wafer 14.

Three such liberated atoms 30, 32, and 34 are shown FIG. 4. Also shownin FIG. 4 is the metal target 22, a cross section of the collimator 26,and the wafer 24. The wafer 24 contains a number of contact holes 10that are to be filled with the metal atoms liberated from the metaltarget 22. The atom 30 has an angle incident to wafer 14 that isrelatively shallow. Thus, the atom 30 runs into the walls of thecollimator 26 and does not make its way onto the wafer 14. The atom 32,by contrast, whose angle incident to the wafer 14 is relatively steep,e.g., substantially perpendicular, compared to that of the atom 30,passes though the collimator 32 and is deposited onto the wafer 14. Ofcourse, the atom 34 traveling perpendicularly to the wafer 14 passeseasily through the collimator 26 and impacts the wafer 24. Generally,the size of the hexagonal holes and the thickness of the collimator 26,i.e., the aspect ratio of the collimator's holes, determine whether anatom traveling at a non-perpendicular angle relative to the wafer 24impacts the wafer 24.

Selecting liberated atoms having substantially perpendiculartrajectories through this means assists in filling the bottoms of thecontact holes 10. Without some means of producing atoms that travelsubstantially perpendicularly to the sides of the contact holes 10,atoms having shallow trajectories would accumulate on the sides of thecontact holes 10, thus leaving the bottom of the contact holes 10 whollyor partially unfilled. However, using the collimator 26, atoms havingundesired trajectories accumulate on the walls of the collimator 26instead of on the walls of the contact holes 10. In fact, in an idealcollimated process, the aspect ratios of the holes in the collimator 26are roughly the same as the aspect ratios of the contact holes 10 thatare to be filled. Accordingly, metal formed by collimated depositionproduces a contact 36 with improved metal coverage, as shown in FIG. 5.While commercial metal sputterers with collimators 26 exist, standardmetal deposition chambers can be modified to include a collimator 26 byspacing the metal target 22 from the wafer 24 and by adequatelyharnessing the collimator 26 to the metal sputtering chamber 20.

Although collimation produces contacts that have better metal coverageat the bottom of the contact holes 10, the technique does suffer fromthe drawback that the inside walls of the collimator 26 become coatedwith atoms having shallow trajectories. Over time, the build-up on thewalls changes the effective trajectory of the atoms that are able toproceed to the wafer surface and affects the profile of the metal filmso deposited. Once this build-up becomes severe, the collimator 26 iscleaned or changed.

To facilitate the collimation process, a wafer having a test structuremay be provided to monitor this build-up. In one embodiment, a testwafer is included along with actual production wafers during the metalsputtering process to provide a convenient means for monitoring processvariations, such as metal step coverage, on the production wafers.However, the test structure may be formed at one or more locations on aproduction wafer to monitor the metal step coverage on the productionwafer. Referring to FIG. 6, such a wafer is shown and designated by areference numeral 40. The wafer 40, which may be a production wafer or atest wafer, may contain one or more trenched structures 42, one of whichis shown in the exploded view of FIG. 6.

The trenched structure 42 includes a plurality of trenches 44, asillustrated in FIG. 7. To form the trenched structure 42, an oxide layer46 is deposited onto a wafer 40. Next, the trenches 44 are etched, e.g.,anisotropically, into the oxide layer 46. The trenches 44 are etchedsuch that the height of the trench 44 divided by the width of the trench44 substantially equals or exceeds the aspect ratio of the contact holes10 that are used in the process to be monitored. In this way, thetrenched structure 42 on the wafer 40 mimics an important parameter ofthe contacts holes 10 in the actual process, i.e., their aspect ratio.In this embodiment, the trenches 44 are approximately two microns inheight.

The metal 50 whose variation is to be measured is sputtered using acollimated process as described previously onto the surface of the wafer40, as shown in FIG. 8. Because the trenches 44 have approximately thesame aspect ratio as the contact holes 10 on a production wafer, themetal 50 assumes a shape inside of the trenches 44 that mimics that of acontact hole 10 in the production wafer.

The electrical resistance of the metal 50 on a particular trenchedstructure 42 is measured across the trenched structure 42 between theterminals 52 and 54. It should be noted that individual trenchedstuctures 42 on the wafer 40 are advantageously electrically isolatedfrom one another so that metal on the intervening surface of the wafer40 does not influence the results of the resistance measurement. Theterminals 52 and 54 on each trenched structure 42 are advantageouslylocated such that they form a line that is substantially perpendicularto the trenches 44. The number of trenches 44 between the terminals 52and 54 could be numerous, perhaps well into the thousands.

By measuring the resistance between the terminals 52 and 54, the stepcoverage of the metal 50, particularly the amount of metal at the bottomof the trenches 44, can be monitored. This is true because the thicknessof the metal 50 over all surfaces of the trenched structure 42, i.e.,the surfaces 56, 58, and 60, affect the resistance measurement. However,experience from the use of the collimated sputtering technique revealsthat the thickness of the metal 50 at the top surfaces 56 is notexpected to vary much and, therefore, will not affect the resistancemeasurement. However, as previously discussed, build-up on the sides ofthe collimator 26 can cause, in particular, changes in metal depositionat the side and bottom surfaces 58 and 60 of the trenches 44. Therefore,the thickness of the metal 50 at the side and bottom surfaces 58 and 60should be the dominate factor in the resistance measurement.Accordingly, the resistance measurement can be used to monitor therelative thickness of the metal 50 at the bottom of trenches 44.

An increase in the resistance over a baseline value for a metal thatexhibits proper step coverage indicates that the metal 50 is undesirablythin and, thus, suggests that the step coverage is inadequate. This istrue because thinner metal is higher in resistance than thicker metal.By monitoring the resistance measurement, the process engineer can knowwhen the metal deposition process is no longer properly filling thecontact holes 10. The resistance of the trenched structure 42 may beaccomplished using several means, including utilization of probe needlesconnected to an ohmmeter. Likewise, a four-point probe methodology couldalso be used. To monitor the sputtering process as a function of time,and to establish a baseline resistance value for a properly sputteredmetal, the terminals 52 and 54 of different trenched structures 42 onthe wafer 40 are spaced by the same distance, i.e., that they span thesame number of trenches 44. Also, to ensure that the trenched structure42 dominates the resistance measurement, the trenches 44 areadvantageously sufficiently long in comparison to the distance betweenthe terminals 52 and 54.

As already noted, the disclosed method is designed to be used “in line”such that the test or production wafers 40 with the trenched structures42 etched into them can be inserted in the metal sputtering productionstep along with production wafers that contain no trenched structures.However, a test wafer 40 can be run without the presence of productionmaterial, and the sputtering process might be monitored instead on abi-weekly basis, for example.

An alternative embodiment allows for the metal 50 to have its resistancemonitored in a direction parallel to the trenches 44. For ease ofdiscussion and illustration, like reference numerals are used to referto similar elements of this alternative embodiment. As shown in FIG. 9,a similar trenched structure 42 is used as in the first embodiment,although for this embodiment the ends of the trenched structure 42 willbe described in more detail. Using the same etch as is used to form thetrenches 44 in the oxide 46, an area 62 of the oxide 46 is etched ateach end of the trenches 44. The actual physical area of the areas 62can vary according to the number of trenches 44 to be tested, but shouldbe at least big enough to allow a resistance measuring probe or theterminals 52 and 54 to be seated into the two areas 62.

The metal 50 desired to be monitored is deposited over the entiretrenched structure 42. Then, the metal 50 is removed from the top of thestructure 42, leaving metal 50 only in the trenches 44, as illustratedin FIG. 10, and at the bottom of the two areas 62. The metal 50 at thebottom of the area 62 acts as bond pads for the resistance measuringprobes. The metal 50 can be removed from the top surface 56 of thetrenched strructure 42 through Chemical-Mechanical Polishing (CMP) orother suitable process. With the structure 42 so completed, resistancemeasuring probes or the terminals 52 and 54 are placed into the twoareas 62 at the ends of the trenches 44 and the resistance is measuredin the same manner as described above. As shown in FIG. 10, because themetal 50 only remains on the side and bottom surfaces 58 and 60 of thetrenched structure 42, this metal 50 dictates the measured resistance ofthe trenched structure 42. Because changes in the collimated sputteringprocess typically only affect the amount of the metal 50 on the side andbottom surfaces 58 and 60, changes in the measured resistance of thetrenched structure 42 correlate to the thickness of the metal 50 on theside and bottom surfaces 58 and 60, thereby providing a monitor thatwill allow the process engineer to determine when and if the collimatedsputtering process has been adversely affected.

Although the embodiments described herein are particularly useful formonitoring the deposition of sputtered metals by collimated means, theteachings may also be used to measure step coverage of a conducingmaterial deposited by other means.

What is claimed is:
 1. A method for monitoring step coverage of aconductive material to be deposited onto a production wafer containingcontact holes having a given aspect ratio, the method comprising thesteps of: (a) placing the production wafer into a deposition chamber;(b) placing a test wafer into the deposition chamber, the test waferhaving a plurality of parallel trenches, the plurality of trencheshaving an aspect ratio substantially identical to the given aspect ratioof the contact holes, wherein each end of the plurality of trenchesterminates in a respective common area; (c) disposing the conductivematerial into the plurality of parallel trenches and into the commonareas; (d) coupling two terminals to the conductive material in therespective common areas; and (e) measuring a resistance between the twoterminals.
 2. The method, as set forth in claim 1, wherein step (c)comprises the steps of: disposing the conductive material onto a topsurface of the test wafer and covering the trenches and common areas;and removing the conductive material from the top surface of the testwafer.
 3. An apparatus for monitoring processing variations of aconductive material relative to a recessed feature, the apparatuscomprising: a substrate having a test structure having a plurality ofparallel trenches, the plurality of trenches having an aspect ratiocorrelative to a given aspect ratio of the recessed feature, whereineach end of the plurality of trenches terminates in a respective commonarea, and wherein the conductive material is disposed in the pluralityof trenches and in the common areas; and two terminals coupled to theconductive material on the test structure.
 4. The apparatus, as setforth in claim 3, wherein the two terminals are coupled to therespective common areas.
 5. An apparatus for monitoring processingvariations of a conductive material relative to a recessed feature, theapparatus comprising: a substrate having a test structure having aplurality of parallel trenches, the plurality of trenches having anaspect ratio correlative to a given aspect ratio of the recessedfeature, wherein each end of the plurality of trenches terminates in arespective common area, and wherein the conductive material is disposedin the plurality of trenches and in the common areas; two terminalscoupled to the conductive material in the respective common areas; and aresistance measuring device coupled to the two terminals.